Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity.
The advent of solid state diodes that emit an infrared laser beam has expanded the useful applications of infrared-sensitive photographic elements. These include making prints from computer assisted tomography scanners, various graphic arts products that are exposed by diode lasers, and infrared-sensitive false color-sensitized photographic materials as described in U.S. Pat. No. 4,619,892 of Simpson et al.
Dyes for sensitizing silver halide to infrared radiation have been known for some time. The most common infrared sensitizing dyes are tricarbocyanine dyes, for example, those described in U.S. Pat. No. 4,536,473. Many prior art infrared sensitizing dyes, however, are of limited utility for use in an element that is to be exposed to an infrared laser diode. Some dyes exhibit a broad absorbance spectrum, giving the silver halide unwanted sensitivity in regions of the spectrum other than the emission wavelength of the laser diode. This can result in poor image separation (or color seParation) between the layers of an element having multiple infrared-sensitive layers, such as a false color-sensitized material as described in U.S. Pat. No. 4,619,892. Some dyes do not absorb at a deep enough wavelength to impart good photographic speed at the emission wavelength of the laser, which can range up to 900 nm, or, if they do, exhibit an undesirably broad spectral absorption. Another problem with many known infrared sensitizing dyes is the poor stability of the sensitized silver halide emulsion during keeping. Therefore, it would be desirable to provide sensitizing dyes for silver halide that offer deep and narrow sensitization in the infrared with good keeping stability.